It is known that the so-called wholly aromatic polyamide resins have higher softening and melting points then those of aliphatic polyamide resins, and exhibit desirable physical and chemical properties, such as: excellent heat-resisting properties, for example, a high ratio of mechanical strength at an elevated temperature to that at a room temperature, superior stabilities in dimension and shape at an elevated temperature, and a high resistance to thermal decomposition; high resistances to various chemicals; superior electrical properties, for example, a high dielectric breakdown strength, and superior mechanical properties, for example, a high tensile strength and high Young's modulus. Also, it is known that the wholly aromatic polyamide resins have high orienting and crystallizing properties. Accordingly, the wholly aromatic polyamide resins are suitable as a material for producing filaments, fibers and films having a high heat resistance, a superior flame-retarding property, and high tensile strength and Young's modulus.
However, the conventional filaments or fibers consisting of the wholly aromatic polyamide exhibit a disadvantage in that it is difficult to dye the filaments or fibers by conventional dyeing procedures, due to the high degrees of orientation and crystallinity thereof. Accordingly, the conventional wholly aromatic polyamide filaments or fibers are utilized, in the non-dyed form, as electricinsulating material for motors and transformers, or as industrial materials for producing filter bags or heating tubes, which materials are not required to be colored. However, the conventional wholly aromatic polyamide filaments or fibers are not used as materials which are required to be beautifully colored, for example, materials for clothing. Accordingly, it is desirable to provide a new type of aromatic polyamide material which is suitable for producing the filaments or fibers which can be easily dyed with conventional dyes.
Generally, an artificial polymer fiber having high degres of orientation and crystallinity of the polymer molecules exhibits a poor dyeing property. In other words, the dyeing property of the artificial polymer fiber can be improved by decreasing the degrees of orientation and crystallinity of the polymer molecules in the fiber. Therefore, in the case of the wholly aromatic polyamide fiber, various approaches for impvoring the dyeing property thereof, by introducing a functional radical having a high degree of affinity to dyes into the polymer molecules, or by blending the aromatic polyamide with a polymer having a high degree of affinity to dyes have been proposed.
Japanese Patent Application Publication No. 45-34776(1970) disclosed a method for producing filaments or fibers from a modified aromatic polyamide copolymer which contains repeating units having a functional radical which exhibits a high degree of affinity to dyes, for example, sulfonic acid radical, tertiary amino radical and quaternary ammonium radical. Also, each of Japanese Patent Application Publications Nos. 49-32658(1974), 49-32659(1974), 49-33594(1974) and 50-8106(1975) disclosed a method for producing filaments or fibers from a composition containing the above-mentioned modified aromatic polyamide copolymer and a non-modified wholly aromatic polyamide. However, it was found that the above-mentioned fibers or filaments exhibited a significantly poor heat resisting property, which is very important when they are used practically, while the filaments or fibers exhibited an enhanced dyeing property.
For example, Japanese Patent Application Publication No. 49-32659(1974) disclosed an aromatic polyamide composition comprising a non-modified wholly aromatic polyamide and a modified aromatic polyamide containing, as an indispensable comonomer, a sulfone-substituted xylylene diamine, and a method for producing filaments or fibers from the above-mentioned aromatic polyamide composition. According to the process of the above-mentioned publication, the filaments produced from the aromatic polyamide composition were heat treated in a relaxed condition, at a temperature of 250.degree. C., for 20 minutes, so as to reduce the thermal shrinkage of the filaments. The resultant filaments exhibited an improved dyeing property. However, in spite of the above-mentioned relaxing operation, the relaxed filaments exhibited an undesirably large shrinkage of more than 30% at a temperature of 300.degree. C. The above-mentioned publications contained no example in which the aromatic polyamide filaments are produced from the compositions at a high temperature of 300.degree. C. or more. Also, it was found that the shaped articles such as filaments and films, made of the compositions disclosed in the above-mentioned publications tended to readily shrink, decompose, discolor or deteriorate at a high temperature of 300.degree. C. or more.
Accordingly, it is clear that the above-mentioned conventional aromatic polyamide compositions are not suitable as materials for producing the shaped articles which are highly resistant to a high temperature of 300.degree. C. or more. This is because the sulfone radical or its alkali metal, or alkaline earth metal salt radical, in the compositions has a poor thermal stability.
Furthermore, it is known that an artificial polymer fiber having an improved dyeing property can be prepared from a polymer which exhibits a poor dyeing property by surrounding a fiber core, consisting of the polymer having a poor dyeing property and extending along the longitudinal axis of the fiber, with a sheath consisting of another polymer having a satisfactory dyeing property. However, the production of such a core-in-sheath type composite fiber is very complicated and expensive. Therefore, if it is possible, it is very desirable to provide a core-in-sheath type fiber having a satisfactory dyeing property which has been converted from a simple aromatic polyamide fiber comprising a single aromatic polyamide material. However, such type of aromatic polyamide fiber has never been known.
Moreover, it is known that a conventional rayon fiber and polyacrylonitrile fiber which have been produced by means of a wet spinning process, have a sheath portion and a core portion embedded in the sheath portion and extending along the longitudinal axis of each fiber. However, in such fibers, the sheath portion is dense and exhibits a poor dyeing property, while the core portion is loose and exhibits a satisfactory dyeing property. Therefore, the sheath portion causes the dyeing property of the fiber as a whole to be less than that of the core portion. Accordingly, from the point of view of dyeing property, it is preferable to remove the sheath portion from the fiber.